1. Field of the Invention
This application relates to oscillator circuits and more particularly to suppression of noise in oscillator circuits.
2. Description of the Related Art
A voltage controlled oscillator (VCO) may be implemented using LC tank circuits. The oscillating frequency of the VCO is proportional to √{square root over (1/LC)}. Thus, it is common to use variable capacitance circuits to vary the capacitance (C) and thus the output frequency of the VCO. Voltage controlled oscillators are commonly found in such applications as phase-locked loop circuits. FIG. 1 illustrates a high level block diagram of a phase-locked loop 100. In the phase-locked loop 100 an input signal 101 and a feedback signal 103 are coupled to a phase/frequency detector 105 that determines the phase/frequency difference between the input signal 101 and the feedback signal 103. That difference is supplied to a loop filter 104, which supplies a control voltage on node 118 to vary the output of the VCO 110 in accordance with the control voltage.
An exemplary VCO circuit is shown in FIG. 2. The exemplary VCO circuit shown in FIG. 2 is an LC tank circuit and includes an inductor 122, a variable capacitance circuit 124, and a gain circuit 126. The output of the VCO can be varied by varying the capacitance circuit 124 in accordance with the control voltage supplied on node 118 (FIG. 1). One variable capacitance circuit utilized in a VCO is described in U.S. Pat. No. 6,147,567, issued Nov. 14, 2000, naming Welland et al, as inventors, which is incorporated herein by reference.
The variable capacitance circuit 124 can have nonlinear capacitances associated both with elements that are used to vary the capacitance and the amplifier circuits. The nonlinear capacitance varies with the voltage seen by the nonlinear capacitor. The nonlinear capacitance can cause the instantaneous output frequency of the VCO to vary as a function of VCO amplitude, which can result in undesirable jitter or noise associated with the output of the VCO. It would be desirable to provide a way to compensate for nonlinear capacitance found in variable capacitor structures to improve oscillator performance.